The invention relates to a method for selecting a modulation detector in a receiver.
When information is transferred over a radio channel, a signal to be transmitted must be modulated. The purpose of modulation is to render the signal such that it can be transmitted at a radio frequency. One requirement of a good modulation method is, for example, that it enables the largest possible amount of information to be transferred on the narrowest possible frequency band. Depending on the use, other features can also be stressed. In addition, modulation must be such that it causes as little interference to a neighbouring channel as possible.
One modulation method is π/4-DQPSK (π/4-shifted, Differential Quatemary Phase Shift Keying) modulation. This modulation method comprises eight phase states but only four phase shifts. The allowed phase shifts (symbols) are ±π/4 and ±3π/4. Each phase shift corresponds to two bits to be transmitted. In other words, a digital signal modulates a carrier in two-bit sequences in such a manner that a given phase shift corresponds to each two-bit combination during each symbol sequence. A symbol sequence refers here to a signal sequence which is used for transmitting two bits. The phase shifts which correspond to bit combinations 00, 01, 10 and 11 are π/4, 3π/4, −π/4 and −3π/4. For example, the symbol frequency employed by the Terrestrial Trunked Radio (TETRA) is 18 kHz, whereby the bit frequency is 36 kHz.
When a signal is received, it has to be demodulated, i.e. the bits that are modulated to the signal have to be detected by a detector in order to find out the information included therein. A receiver may comprise a plurality of detectors which are optimized for various channel conditions. In some conditions, a channel equalizer may also be needed. Selection of a detector to be used is generally implemented such that the detectors operate simultaneously and each produces a commensurable error-metric value, on the basis of which the detector that is best suited for the conditions can be selected.
The above-described arrangement has a drawback that as the detectors operate simultaneously, a considerable amount of computational power is required for calculating the detector algorithms. In particular, a channel equalizer that is possibly included in the detector requires heavy computational power.